The 25-year-old striker has signed an 18-month deal after securing his release from the Sky Bet League One promotion hopefuls, with manager Stephen Robinson promising more attacking reinforcements to come.
Main came through the youth ranks at Sunderland but joined Darlington where he made his league debut aged 15. His performances for the Quakers alerted a number of Premier League clubs, with Fulham and Newcastle both linked with the player. But Tony Mowbray’s Middlesbrough eventually signed him and Main played in the Championship as a teenager.
He moved to Doncaster where he netted 11 times in 34 starts and had a loan spell at Oldham which ended shortly before Robinson took charge there.
Portsmouth signed him in the summer of 2016 and he hit five goals in 14 appearances before injury ended his first season in October.
With Portsmouth vying for promotion to the Championship, Main has been unable to dislodge goalscorers Brett Pitman and Ollie Hawkins and has been allowed to leave to find first-team football.
Main told Motherwell’s website: “A few weeks ago I heard about the interest and it’s been back and forth with the club and myself but finally we’re here and I can’t wait to get started.
“There were a few options but I felt this challenge, coming up here and testing myself in a new environment, is something that really appealed to me.”
Robinson, who saw Moult depart for Preston in a £500,000 deal after netting 14 times this season, added: “I’ve tried to sign him a number of times before and I am excited to finally be working with him.
“I’ve brought him here to bolster our attacking options and get amongst the goals. He’s a strong, sharp, tenacious player with an eye for goal. If you look back on his career, he’s scored all different types.
“I am still looking to add more forward players to create real competition in that area of the field.”